1. Field of the Invention
This invention relates to an apparatus for liquid-jet recording by jetting liquid droplets by heat generation to perform recording with the liquid droplets.
2. Description of the Prior Art
FIG. 1(a) is a cross-sectional plan view showing one example of the conventional liquid-jet recording head, and FIG. 1(b) is a cross-sectional view along the line A--A of FIG. 1(a) where heat-generating means composed of electro-thermal transducing parts 2 (as will be hereinafter referred to as "heat-generating parts) and electroconductive parts 3 are formed on a substrate 1, and a protective film (not shown in the drawings) is formed thereon. Each of the heat-generating parts 2 is partitioned by grooved plates 4 to form a liquid passage 5 having a thermal action chamber in which the heat energy generated by said heat-generating means acts on a liquid, and a liquid supply chamber 6. A discharge outlet 7 is provided at one end of the liquid passage 5, and the liquid is jetted from the discharge outlet. The liquid to be jetted is supplied through a liquid supply pipe 8 provided at the opposite side of the discharge outlet 7 across the heat-generating means to fill the liquid supply chamber 6 and the liquid passage 5.
The liquid can be jetted from the discharge outlets 7 by the heat generated at the heat-generating parts 2. The heat is generated by applying a predetermined pulse voltage to the electroconductive parts 3 connected with the heat-generating parts 2. When the voltage is applied thereto, the liquid near the heat-generating parts 2 undergoes rapid state changes accompanied by bubble formation by the generated heat energy, and the bubbles rapidly grow within the liquid passage 5. The liquid on the side of discharge outlet 7 is pushed out of the discharge outlet 7 rapidly by the generated pressure to form liquid droplets. The liquid droplets deposit onto a recording material to perform recording. When the applied voltage is turned off, the bubbles are rapidly contracted and vanished.
In such a liquid-jet head, a protective film is generally provided so that the electro-thermal transducing means having the heat-generating parts 2 and the electroconductive parts 3, i.e. a heat-generating means having a resistor and at least one pair of electrodes electrically connected with the resistor as counterposed to the heat generating part of the resistor may be protected from any contact with the liquid.
FIG. 2 is a cross-sectional view of detail of a the heat-generating part 2 of the liquid-jet recording head shown in FIG. 1(b), where a resistor 9 and an electrode 10 are formed on the substrate 1 and the part where there is only resistor 9 corresponds to the heat-generating part 2 in FIG. 1 and the part of the resistor 9 and the electrode 10 that overlap corresponds to the electroconductive part 3 in FIG. 1. The resistor 9 and electrode 10 comprising the heat-generating means is protected from a liquid 12 by a protective film 11.
The resistor 9 and electrode 10 have a risk of deterioration, changes in resistance or breaking-down due to chemical reactions such as oxidation reaction, electrolysis, etc., when brought into contact with the liquid 12. Thus, the protective film 11 is provided to prevent such a risk. The protective film 11 has no problem, so long as it is perfect, and the resistor 9 and electrode 10 are completely separated from the liquid 12, and a long life of the resistor 9 can be ensured.
However, it is actually very difficult to form such an ideal protective film. In the ordinary manufacturing process, fine defects 13 of less than a few microns are inevitably formed on the protective film 11, as shown in FIG. 2. Furthermore, defects 13 are also formed in the protective film 11 due to the thermal stress caused by the heat generation at the heat-generating part 2 of the resistor 9 or impacts, etc. caused by generation and vanishing of bubbles as described above.
FIG. 3 shows one example of a circuit structure of the conventional liquid-jet recording head, where the resistors 9 of the heat-generating part 2 and switching transistors 14 are connected with one another in series, and a plurality of the series connections are connected with one another in parallel. The higher voltage side of a power source 15 is connected with the resistors 9, and its lower voltage side (ground side) is connected with the switching transistors 14. Suppose that the ground potential is Vg and the potential on the resistor 9 side is Vh.
FIG. 4 is a diagram showing changes in voltage at the part A on the switching transistor 14 side of any of the resistors 9, where it is shown that a pulse-form voltage is applied to the resistor 9 by the on-off action of the switching transistor 14. The abscissa shows time, and the ordinate shows voltage.
In the circuit structure shown in FIG. 3, the potential Vink of liquid 12 (see FIG. 2), which may be hereinafter referred to merely "Vink", is substantially equal to Vh due to the defects formed on the protective film, when a given switching transistor 14 is in off state. The potential of the corresponding resistor 9 is Vh on the whole, and thus there is no difference in potential from the liquid 12. On the other hand, when the switching transistor 14 is on, an electric current passes through the resistor 9 to generate heat, and the potential at the part A of resistor 9 is lowered nearly to the ground voltage Vg at the same time. However, the potential Vink of liquid 12, may still remain nearly at Vh, and thus a potential difference of substantially Vh - Vg develops between the liquid 12 and the part A of resistor 9.
When the potential of liquid 12 is high and that of resistor 9 is low, as given above, it is known that an electric current is liable to pass to the resistor 9 through the defects 13 as shown in FIG. 2, and consequently electrochemical reaction is promoted between the electrode or the resistor and the liquid, and ultimately the resistor 9 will be damaged around the defects 13 and broken down. This is remarkable particularly when the defects 13 exist at the part A of resistor 13.
The rate of the reaction greatly depends on the species of resistor 9 and electrode 10, the heat-generating temperature of resistor 9, species of ions in the liquid, etc. When a defect 13 is formed at a heat-generating part 2, the heat-generating part 2 of resistor 9 is usually damaged and broken down only by about 10.sup.5 to 10.sup.6 applications of pulse voltage, through practically a durability to withstand at least about 10.sup.8 applications of pulse voltage is required.
Thus the presence of defects 13 on the protective film 11 shortens the life of heat-generating part 2 of resistor 9, and consequently shortens the life of the head, because breakage of only one resistor can terminate the life of the head, even if the head is of full-line multiorifice type. However, it is very difficult to completely remove the defects 13 as already described above. An increase in the thickness of protective film 11 must be avoided from the viewpoint of thermal conductivity. Thus in the production of the conventional recording heads, some heads with a short life are unavoidably involved, and the product reliability is considerably reduced.